The objective of this research is to develop ultrasound propagation algorithms and tissue models that mimic ultrasound scattering behavior in typical breast specimens for use in studying breast imaging techniques, and then to employ these models and algorithms to refine and evaluate adaptive focusing methods that correct for ultrasound beam aberration caused by propagation through breast inhomogeneities. The specific aims are to: 1) acquire high-resolution magnetic resonance imaging data throughout the volume of the breast and segment the data into tissue types with acoustic properties characterized by random processes, 2) calculate acoustic propagation in three dimensions through the modeled volume of the breast to determine the aberration produced by inhomogeneities in the breast, 3) perform pulse-echo measurements of aberration using the same specimens to validate the modeling of the breast and the calculation of aberration, and 4) simulate high-resolution b-scan images by using adaptive focusing that compensates for aberration. The segmented data will be used to develop realistic numerical models for calculations of propagation. Calculations of pulse propagation through the breast models will use three-dimensional k-space and fast multiple methods. Propagation through the models will be used to simulate measurements that are repeatable and easy to alter, allowing for efficient refinement of aberration-correction methods. Aberration will be determined using two new algorithms. In one algorithm, the aberration is estimated using cross spectra of pulse-echo signals obtained from a set of focuses in an isoplanatic region. In the other algorithm, aberration is estimated using cross correlation of echoes obtained from broad-beam illuminations produced by virtual sources. Variations of the parameters that govern these algorithms will be explored to optimize algorithm performance. Simulated point-reflector echoes received through the aberration path will be used to compute the true aberration for comparison with the aberration found using the two algorithms. The true focus achieved in each of the cases will be described by calculations and also by hydrophone measurements. Focus characteristics as well as image resolution will be evaluated with respect to breast morphology in the propagation paths. Arrival time fluctuations, waveform shape changes, and statistics of distortion will be determined for both calculated and measured results. Also, the size of the region over which aberration can be satisfactorily compensated with a single set of parameters will be determined. Focus characteristics and image resolution will be carefully evaluated and critically compared to available geometric and adaptive focusing techniques. Quantitative conclusions about the effects of aberration in ultrasound imaging of the breast and the performance of the two aberration correction algorithms will be developed. As a result of this research, adaptive focusing techniques using aberration correction will yield improved resolution that will significantly increase the capability of ultrasound b-scan imaging to distinguish between normal and diseased breast tissue and to determine the severity of breast disease in circumstances not now possible with ultrasound.